1. Field of the Invention
The present invention relates to a standard signal generator for calibrating a converter of an electromagnetic flowmeter.
2. Description of the Related Art
FIG. 7A is a block diagram illustrating the configuration of an electromagnetic flowmeter of the related art. The electromagnetic flowmeter includes a detector 1 and a converter 2. The detector 1 includes an excitation coil 10 that generates a magnetic field, and a measurement tube 11 adapted for placement in the magnetic field generated from the excitation coil 10, for detecting an electromotive force produced by a fluid under measurement flowing through the magnetic field and outputting a flow rate signal that is proportional to the flow velocity of the fluid. The converter 2 supplies an excitation current such as that illustrated in FIG. 7B to the excitation coil 10 of the detector 1, and converts the flow rate signal input from the detector 1, such as that illustrated in FIG. 7C, into an analog signal or a digital signal indicating the flow rate or flow velocity of the fluid.
The flow rate signal input from the detector 1 to the converter 2 is a minute signal on the order of microvolts (μV), which may lead to deterioration in measurement accuracy due to aging of electrical components used in the converter 2. To address the deterioration, the electromagnetic flowmeter is subjected to periodic calibration in the following way with the use of a standard signal generator (hereinafter referred to as a “calibrator”) at the site where the electromagnetic flowmeter is installed (see Japanese Unexamined Utility Model Registration Application Publication No. 6-43521, Japanese Unexamined Utility Model Registration Application Publication No. 6-69743, and Japanese Unexamined Patent Application Publication No. 7-146165).
In the calibration operation, first, in place of the detector 1, a calibrator 3 having a configuration illustrated in FIG. 8A is connected to the converter 2. The calibrator 3 includes an input circuit 30 that receives an excitation current input from the converter 2, such as that illustrated in FIG. 8B, a central processing unit (CPU) 31 that generates a reference flow rate signal, an output circuit 32 that outputs the reference flow rate signal generated by the CPU 31, a setter/indicator 33 used for the setting of the calibrator 3 and the visual indication of information to a calibration operator, a power supply circuit 34, and a battery 35. The calibration operator sets, in the calibrator 3, information on the type of the converter 2 and a flow velocity value at a calibration point by using the setter/indicator 33.
The CPU 31 of the calibrator 3 outputs a reference flow rate signal corresponding to the set flow velocity value in synchronization with excitation currents input from X and Y terminals of the converter 2 via the input circuit 30. The reference flow rate signal is input to the converter 2 via the output circuit 32 as a signal illustrated in FIG. 8C. The calibration operator checks data output from the converter 2 in accordance with the reference flow rate signal to determine whether the degree of measurement accuracy of the converter 2 is tolerable. The converter 2 is re-adjusted, if necessary, in accordance with the check result.
In the calibration operation with the use of the calibrator 3 described above, a commercial power supply may not be available at the site where the electromagnetic flowmeter is installed. To perform a calibration operation even at such a site, the battery 35 is used as a power supply for the calibrator 3 and voltages necessary for the components of the calibrator 3 are generated by using the power supply circuit 34. The voltage of the battery 35 is fed into an analog-to-digital (A/D) converter of the CPU 31 and the setter/indicator 33 provides a visual indication in accordance with the remaining capacity of the battery 35 (FIGS. 9A to 9D).
In an example illustrated in FIGS. 9A to 9D, the remaining capacity indication is provided using four stages: a remaining battery capacity of 80 to 100% (FIG. 9A), a remaining battery capacity of 30 to 79% (FIG. 9B), a remaining battery capacity of 10 to 29% (FIG. 9C), and a remaining battery capacity of less than 10% (FIG. 9D). The operation is interrupted if the battery 35 runs down during the operation. Thus, when the battery voltage decreases to around a level at which the calibrator 3 fails to operate normally (in the example illustrated in FIG. 9D, at a remaining capacity of less than 10%), the remaining capacity indication is caused to flash at intervals of, for example, one second (to indicate a remaining battery capacity alarm) to advise replacement of the battery 35.
A traditional calibrator uses a battery as a power supply and once the battery is depleted at the site, the calibration operation is difficult to perform. To address this difficulty, a method is available for generating a power supply voltage for a calibrator by utilizing an excitation current supplied from a converter. It should be noted that, in a two-wire electromagnetic flowmeter, an excitation current supplied from a converter is so small (less than or equal to approximately 25 mA) as to be used as a power supply of a calibrator. In a two-wire electromagnetic flowmeter, therefore, a battery is also used as the power supply to allow the calibrator to be driven by battery when the converter connected to the calibrator is a converter of a two-wire electromagnetic flowmeter.
In a case where both a power supply voltage generated from an excitation current and a power supply voltage supplied from a battery are available as power supply voltages for a calibrator, the calibrator can operate without a battery if the converter connected thereto is a converter of a four-wire electromagnetic flowmeter since the excitation current supplied from the converter is sufficiently large. However, without a battery, a remaining battery capacity alarm such as that described above is activated, which results in a user who uses only a four-wire electromagnetic flowmeter being unnecessarily bothered.